Wireless data communication networks of various types, including digital cellular systems, Wireless Local Area Networks (WLANs) and even personal area networks such as Bluetooth are increasingly viewed as an ideal connectivity solution for many different applications. These can be used to provide access to wireless equipped personal computers within home networks, mobile access to laptop computers and personal digital assistants (PDAs), as well as for robust and convenient access in business applications.
Indeed, it is estimated at the present time that approximately 10% of all laptop computers are shipped from the factory with wireless interface cards. One estimate is that this ratio will increase to 30% within the next two years. Certain microprocessor manufacturers, such as Intel, have incorporated wireless capability directly into processor chip platforms. These and other initiatives will continue to drive the integration of wireless equipment into computers of all types.
It is actually already possible in some cities to find “hot spots” where one can obtain connectivity to many different networks at the same time. Unfortunately, having tens, if not hundreds, of closely spaced networks means that interference becomes a problem. That is, although the most emerging wireless standards provide for robust signaling in the form of spread spectrum radio frequency modulation, or using Code Division Multiple Access (CDMA) over modulated subcarriers, crowding of the radio spectrum still increases noise and therefore decreases performance for all users.
The capacity of CDMA networks which use a frequency reuse factor of one is limited by both intercell and intracell interference. Techniques such as Multi User Detection (MUD) can be used to mitigate intracell interference. Intelligent management of channel power, code words, and time slots (that is, robust Radio Resource Management (RRM)) can also be used.
Of most interest to the present invention, it is also possible to use a directional, or adaptive, antenna to determine the optimal direction in which to transmit and receive signals. The directional antenna focuses the radiated power of such signals, so as to minimize interference with other transmissions.
One technique that can be used to mitigate intercell interference is a directional antenna on the mobile (remote) or so-called User Equipment (UE). To understand the advantages of doing this, consider a situation where adjacent base stations or sectors cause intercell interference on the forward link channels transmitted from Central Base Station Transceivers (BTSs) to the UEs. If these downlink (DL) channels have angular separation between the signals of the desired base station or sector and those of the interfering base stations or sectors, then the directional antenna on the UE can provide some amount of suppression of the interference. The exact amount depends on the angular separation, front-to-back ratio of the antenna, and the beam width of the antenna.
Users in adjacent cells or sectors also cause intercell interference on the uplink (UL) or reverse direction. If the directional antenna can be pointed such that most of the transmitted energy is directed to the desired base station and away from the adjacent cells or sectors, then the antenna can provide intercell interference suppression on the uplink as well. This interference suppression will manifest itself as a reduction of interference at the desired base station receiver.
The use of a directional antenna therefore contributes directly to improvements in link budget. It provides additional antenna gain over a standard omni directional antenna when operated in a directional mode. Depending on the algorithm used for steering, the additional gain can contribute directly to both the uplink and downlink link budgets. The directional antenna also reduces the effects of fading due to local scattering. The directivity of the antenna allows only a portion of the path structure created by the local environment to reach the receiver input, reducing the amount of fading. The required fast fade margin is therefore also reduced.
However, in order for the directional antenna to be most effective, it must be steered in the proper direction for both uplink and downlink. In packet switched Frequency Division Duplex (FDD) systems, the directions for both downlink and uplink typically must be the same since both the UL and DL carrier frequencies are active at the same time. Often, a compromise direction is thus picked to optimize reception in both directions.
However a Time Division Duplex (TDD) system has certain advantages over FDD when it comes to antenna steering:                Because the UE is half duplex, DL and UL pointing directions can be different, allowing an optimum direction to be selected in each case.        Because the DL and UL typically operate on the same frequency, under most conditions, the DL and UL path losses will be the same.        The frame structure of TDD is such that there are non-active time slots available for checking alternate antenna directions and computing the antenna steering metrics.        